With the drastic advancement of the electronic, communication and computer industries these days, camcorders, mobile phones, laptop PCs, etc. have made remarkable progress, and thus the demand for a lithium secondary battery is increasing as a power source for driving such devices.
The cathode active material of the lithium secondary battery mainly includes lithium cobalt oxide (LiCoO2; LCO), and currently commercially available as inexpensive materials having high safety are spinel-type lithium manganese oxide (LiMn2O4; LMO), lithium composite metal oxide (LiMn1/3Co1/3Ni1/3O2; NMC) and olivine-type lithium iron phosphate oxide (LiFePO4).
Among the above cathode active materials, lithium cobalt oxide (LCO) has high energy density, and is mainly used as a power source for small appliances such as mobile phones, laptop PCs, etc. due to problems with transition metal materials and stability problems, but is not suitable for use in a large lithium secondary battery for electric cars requiring higher stability.
On the other hand, spinel-type lithium manganese oxide (LMO) has a high energy density of about 120 to 140 mAh/g, and is known to exhibit excellent thermal stability of a cathode active material itself under overcharge and high voltage conditions due to the structural stability of the material, but has a structural problem in which manganese is dissolved when the battery temperature is increased to about 60° C.
Also, lithium composite metal oxide (NMC) has a high energy density of about 140 to 180 mAh/g, but is disadvantageous in terms of safety due to problems with the transition metals cobalt and nickel. Thus, such conventional materials have lower capacity than expected or are still dangerous, and are very expensive, and hence, inexpensive materials having high safety and high energy density are required in order to commercialize medium- or large-sized batteries.
Currently, a medium- or large-sized lithium secondary battery is required to exhibit high safety, a long lifetime, high energy density and cost-effectiveness, and thus an olivine-type cathode active material including iron is receiving attention. A typical olivine-type cathode active material, namely a lithium iron phosphate compound (LiFePO4), is a cathode active material having superior electrical capacity of about 150 to 160 mAh/g, but has a discharge voltage of 3.2 V to 3.4 V, which is lower than those of other oxide-based cathode active materials and is thus unsuitable for use as a cathode active material for a medium- or large-sized lithium secondary battery requiring high energy density.
In contrast, the same olivine-type compound, namely a lithium manganese phosphate compound (LiMnPO4), has a high discharge voltage of 3.8 V to 4.0 V, similar to those of oxide-based materials, and thorough research thereto is thus carried out in order to improve the characteristics thereof.
Japanese Patent Application Publication No. 2007-119304 discloses a method of preparing a lithium manganese phosphate compound (LiMnPO4) at a low temperature under pressure through precipitation and reduction of Mn(OH)2. However, the obtained lithium manganese phosphate compound (LiMnPO4) cathode active material has a very low electrical capacity of about 40 mAh/g and thus poor electrochemical properties, making it impossible to use industrially.
Also, Japanese Patent Application Publication No. 2007-48612 discloses a method of preparing a lithium manganese phosphate compound (LiMnPO4) by forming a material mixture and then firing the material mixture recovered through spray drying. The obtained lithium manganese phosphate compound (LiMnPO4) cathode active material has an electrical capacity of 92 mAh/g at a current density of 0.25 C, and the lithium manganese phosphate compound (LiMnPO4) containing 15% carbon has 130 mAh/g at a current density of 0.12 C, but the amount of the cathode active material in the cathode is excessively low, to a level of about 63%, from which the electrochemical properties of the cathode active material itself are not regarded as being sufficiently improved.